1. Field of the Invention
This invention relates to improvements in methods and structures for reducing capacitive effects on integrated circuit resistors, or the like, and in a particular embodiment to improvements in optoelectronic current-to-voltage converter sensors and methods for making and using same.
2. Relevant Information
In the construction of integrated circuits, one structure in which resistors are made is a stripe in a layer that is formed during an integrated circuit fabrication process. For example, such resistor may be formed from a doped region in a polycrystalline silicon (polysilicon) layer, one or more of which are typically found in silicon integrated circuit fabrication processes. Such resistors may be of significant lengths, and are usually laid out in a zigzag or serpentine pattern to conserve chip area. The resistance of the resistor is a function of the length and width of the stripe as well as of the levels of dopants within the stripe.
Such resistors can be considered in the same manner as a transmission line, having a distributed resistance and capacitance along its length. The capacitance referred to is generally thought of as a parasitic capacitance between the resistor and the semiconductor substrate above which the resistor is formed, although other capacitances also may have capacitance contributions, depending upon the particular circuit embodiment in which the resistor exists. For example, a capacitance may exist between the various portions or segments of the resistor in its zigzag layout; however, such capacitances are usually of lesser importance than the resistor-to-substrate capacitance mentioned above.
Although examples of such integrated circuit resistors are manifold, one example of such resistor which is widely used is in sensors in optoelectronic applications, such as a current-to-voltage converter sensor in which a feedback resistor, R.sub.F, is connected between the output of an operational amplifier and its inverting input. The cathode of a photodiode is connected to the inverting input of the operational amplifier, and the anode is connected to the non-inverting input of the operational amplifier and to ground.
In this sensor embodiment, photodiode current, I.sub.L, is generated when light is incident on the photodiode. The photodiode current flows through the feedback resistor, R.sub.F, generating an output voltage V.sub.OUT equal to (I.sub.L) (R.sub.F). This circuit configuration, which may be constructed as an integrated circuit on a semiconductor substrate, provides an output proportional to light input, except for offsets due to normal operational amplifier offset and finite operational amplifier gain. The circuit minimizes the absolute voltage across the photodiode, which minimizes photodiode leakage currents, especially at high temperatures. The circuit also minimizes the effect of the photodiode capacitance on speed.
When the operational amplifier and feedback resistor of optoelectronic light-to-voltage converters are integrated onto a semiconductor substrate, typically the feedback resistor has a resistance of up to 16M.OMEGA. and is constructed using a second polysilicon level having a resistivity of 500.OMEGA./.quadrature., for example, in a LinBiCMOS.TM..sup.1 process. A limitation on the sensor speed is the parasitic capacitance between the polysilicon feedback resistor and the substrate. Such parasitic capacitance between the resistor and substrate severely limits the bandwidth of the sensor. FNT .sup.1 LinBiCMOS is a Trademark of Texas Instruments Incorporated
In the past, the general solution for high bandwidth sensors has been to use an external resistor to minimize the parasitic capacitance of the resistor. However, such external solutions increase the size and cost of the sensor.
What is needed, therefore, is a method and apparatus to minimize the effect of the parasitic capacitances of integrated circuit resistors.